The present invention relates to a fraud detection method for authenticating that an object is a human finger. Fingerprint or venous network authentication is one of the techniques used for biometric authentication. An image of an illuminated finger is acquired and analyzed to identify an individual. The most used method consists in acquiring the image of the finger when it is against a surface and in exploiting the optical interactions between the finger and this surface. This is notably the case of the so-called total internal reflection method, better known under the acronym TIR, in which differences in the refractive indices between air and air Human skin to highlight ridges and valleys of fingerprints.
Nevertheless, the need for contact between the finger and the surface poses several problems. The finger must be stationary when acquiring the image to be processed and the process is slow, unsuitable for massive use in places of high traffic, such as airport controls, where the speed of control is important. Moreover, the quality of the image thus acquired depends on the state of the skin of the finger, which may be more or less dry, and the state of the surface, whose repeated contact with fingers can foul it, and therefore requires periodic cleaning.
In addition, the surface contacting the skin of different people, problems of hygiene can arise, exacerbated by the fact that it is the fingers of these people which come into contact with the surface. It may then be necessary to provide equipment for cleaning and/or disinfecting the hands of persons after contact with the surface.
It has therefore been developed devices for acquiring images of fingerprints of at least part of a hand of a user capable of operating without contact. Among the methods developed, acquisition in three dimensions (3D), in which we obtain a 3D model of the finger, is particularly interesting. For example, patent application WO 2009/112717 describes a method in which a three-color light pattern is projected on the surface of the finger and monochrome images acquired in each of the colors projected by a camera makes it possible to reconstitute a model in three dimensions of the finger.
Like any fingerprint acquisition method, it is possible to use decoys, false elements of the human body reproducing the characteristics on the basis of which the analysis of the image is based, such as fingerprints or the venous network of the finger. It is therefore necessary to provide in addition a validation method to validate that the analyzed object is indeed an element of the human body, usually a finger, but it may be the hand as a whole, or another part of the body.
Several methods have been developed to validate or not that an object is an element of the human body, generally making use of various properties of a living finger that are more or less difficult to reproduce. Among these solutions, some aim to highlight the appearance and disappearance of the venous networks as a function of the pressure exerted by the finger. However, these methods cannot be implemented on a contactless sensor.